1. FIELD OF THE INVENTION
The present invention relates to a method of manufacturing ferroelectric crystals, and more particularly to a method of manufacturing ferroelectric crystals, for example, tri-glycine series ferroelectric crystals having characteristics suitable for use in various applications in electronics from heterogeneous crystals.
2. Description of the Prior Art
There are various ferroelectric crystalswwhich are recently found not to be homogeneous such as tri-glycine sulphate, guanidine aluminum sulphate hexahydrate, tri-hydrogen lithium selenite, di-calcium strontium propionate, and their isomorphous ferroelectric crystals. All of these crystals are water soluble. The manufacturing method according to the present invention hereinafter described is applicable to all these ferroelectric crystals. However, the following description of the method according to the present invention will be made with reference to tri-glycine sulphate and its isomorphous ferroelectric crystals only for the sake of simplicity.
Tri-glycine sulphate (hereinafter referred to as TGS), tri-glycine selenate (hereinafter referred to as TGSe), tri-glycine fluoberyllate (hereinafter referred to as TGFB), deuterated TGS (hereinafter referred to as DTGS) and deuterated TGSe (hereinafter referred to as DTGSe) are known as tri-glycine series (hereinafter referred to as TG series) ferroelectrics. All of these ferroelectrics are water soluble crystals having similar crystal structures. Their Curie temperatures are believed to be around 49.degree.C, 22.degree.C, 70.degree.C, 60.degree.C and 34.degree.C, respectively. These crystals are promising in the application to electronic elements such as ferroelectric DC - AC converters, acoustoelectric transducers, infrared ray detectors, memory storage devices and the like. However, since crystals produced by a conventional water solution method have disadvantages as described later, they have seldom been used in practice.
When TG series ferroelectric crystals are employed as electronic elements as described above, it is desirable in practice to produce a single crystal having a b-plane large in area because a b-plate crystal in which the ferroelectric axis (b-axis) is perpendicular to the plane of the plate is employed for such electronic elements.
A b-plate crystal of TG series ferroelectric material can be obtained by producing an idiomorphic single crystal by a rotatory cooling method or circulating method similarly to ordinary water soluble crystals and by cleaving the crystal perpendicularly to the b-axis. However, this method has the disadvantage that the area of the b-plane necessary for technological application is not so large. Accordingly, the following non-stiring cooling method is usually practised for obtaining a large b-plate crystal.
That is, two glass plates are positioned horizontally and parallel with each other. The lower glass plate is provided at its center portion with a recess in which a seen crystal is fixed with its b-axis being directed vertically. Then, the glass plate assembly is submerged in a mother liquid and cooled quietly to grow the crystal in directions perpendicular to the b-axis. By this method a b-plate crystal having a large area of b-plane, of the order of several cm in the direction of the a-axis, can be provided relatively easily.
The appearance of a large size b-plate crystal obdtained as stated above is generally as shown in the central portion of FIG. 1. In FIG. 1, reference numeral 1 designates the seed crystal, and 2 designates the b-plate crystal. Arrows a, b and c at the upper left indicate directions of the crystal axes.
According to the investigation made by the inventors, the entire surface of the b-plate crystal does not have a uniform property. This is the case also for the b-plate crystal obtained by cleaving the aforementioned idiomorphic crystal. According to the results of observations of the growth pattern, cleavability, domain structure, dislocation density, etc., the b-plate crystal seems to be substantially separated into four pairs of regions as indicated by chain lines in FIG. 1, that is, four pairs of sector regions connecting the seed crystal and (001) plane, the seed crystal and (101) plane, the seed crystal and (110) and (110) planes, and the seed crystal and (111) and (111) planes, which will hereinafter be referred to as (001) region, (101) region, (110) region, and (111) region, respectively. If these regions are observed from the view point of lattice defects, (001) region is the one having the least lattice defects, (110) and (111) regions are those having the most lattice defects, and (101) region is the one having intermediate lattice defects.
Furthermore, electrical and dielectric characteristics also show a marked difference from region to region. For example, polarization (P) versus electric field (E) hysteresis loops, i.e. dielectric hysteresis loops measured on a b-plate crystal of TGS show marked difference from region to region as indicated by (a.sub.1), (a.sub.2) - - - (a.sub.8) in FIG. 1. In (111) and (110) regions, the spontaneous polarization P.sub.s is small and the internal biased field E.sub.b is large, and hence the hysteresis characteristic is poor, while in (001) region the spontaneous polorization P.sub.s is large and the internal biased field E.sub.b is small, and hence the hysteresis characteristic is excellent. In (101) region, the spontaneous polarization P.sub.s and internal biased field E.sub.b show intermediate values. For example, on an average P.sub.s (111) = (1/5) - (1/10) .times. P.sub.s (001), P.sub.s (110) = (2/3) - (1/5) .times. P.sub.s (001), P.sub.s (001) = 3.3 .mu. coulombs/cm.sup.2 at room temperature. Consequently, (001) region is among the four regions the most suitable for various applications in electronics. For example, in second harmonic type modulators, the fact that the internal biased field E.sub.b is small is a necessary condition for an element because the internal biased field E.sub.b is a main cause for the offset. In pyroelectric infrared ray detectors, the sensitivity thereof corresponds to the magnitude of the spontaneous polarization P.sub.s.
Furthermore, (001) region is low in density of lattice defects and excellent in its electrical characteristics as compared with other regions. These facts can be seen from the following consideration. The surface energy of a natural face is determined mainly be the state of chemical bond in a cuboid corresponding to the unit cell of the TGS series ferroelectric material. In regard to (001) plane, among the six planes of the unit cell the bond of only one of (001) planes is disconnected. In regard to (101) plane, among the six planes, the bonds of each one of (100) and (001) planes are disconnected. In regard to (110) plane, among the six planes, the bonds of each one of (100) and (010) planes are disconnected. Since the b-axis is the ferroelectric axis in TGS series ferroelectrics, electric charges appear at (010) plane and the electrostatic energy is large. In regard to (111) plane, among the six planes, the bonds of each one of (100), (010) and (001) planes are disconnected. Consequently, the relation between the amounts of surface energies U of natural faces is EQU U.sub.(001) &lt;U.sub.(101) &lt;U.sub.(110) &lt;U.sub.(111)
since the density of lattice defects in a crystal face depends on the surface energy, the density is lowest in (001) face and highest in (111) face. Since a bulk of crystal is the stacking of planes, the relation between the bulk densities n of lattice defects is EQU n.sub.(001) &lt;n.sub.(101) &lt;n.sub.(110) &lt;n.sub.(111)
Also, since the elecrical characteristics depend on the density of defects, in particular impurity ions and charge jogs, the region having the least defects has the best electrical characteristics. Thus, (001) region is the least in point type (impurities, vacancies, etc.), linear type (dislocations), and planar type (domain walls, stacking fault) defects, and, as a result, the best in electrical characteristics. Consequently, it is good to cut out crystals for use as electronic elements from (001) region.
Further, according to measurements by the inventors, (001) region is large in maximum value of the dielectric constant, large in pyroelectric coefficient and piezoelectric constant, and uniform in its characteristics throughout the region.
The abovementioned characteristics are recognized not only in TGS, but also in TGSe, TGFB, DTGS and DTGSe.
However, in a b-plate crystal provided by the conventional method of fabricating single crystals, a region having favorable characteristics is only a portion of the crystal as seen from the above description, and hence there is the disadvantage that practically the yield is very low.